Rapid gas-generating devices or inflators, as they are referred to in the art, have found widespread use. One use is in passive air bag restraint systems in order to reduce the large number of deaths and injuries occurring in automobile accidents annually. Air bags and inflatable belts for passive restraint systems are operatively associated with inflator devices which are generally activated by an inertial switch or sensor which detects rapid deceleration of a vehicle such as that which occurs upon impact between an automobile and another object. When the inertial switch is triggered, it causes an inflator to inflate a collapsed flexible bag or belt quickly which is deployed into a protective position in front of the occupant. The bag or belt must inflate extremely rapidly after the primary impact or collision in order to protect the occupants from injury caused by secondary impact or collision with the interior of the vehicle. In order to meet such criteria, the bag or belt should be fully inflated within about 10-65 milliseconds after inflation has been initiated.
A variety of conflicting design considerations must be taken into account in developing an effective air bag passive restraint system. First, the inflator must be capable of producing and/or releasing a sufficient quantity of gas to the air bag within the time limitation required of a passive restraint air bag system, given the time limitation involved in air bag restraint systems, roughly about 10 to 15 milliseconds for side impact applications and 30 to 65 milliseconds for driver and front passenger applications. Inflators must be capable of filling an air bag in these time frames with 15 to 50 liters of gas for side applications and 60 to 200 liters of gas for driver and front passenger applications. The specific amount and rate of gas generation or release is determined by the required air bag volume and the vehicle structural rigidity which influences the time between primary and secondary impacts.
Other considerations in designing an inflator for a passive air bag restraint system, particularly for automotive applications, include the toxicity and noxiousness of the gas which fills the air bag. That is, the inflator for an automotive air bag must generate or release gas and other materials which meet or surpass certain non-toxicity requirements in order to protect the occupants in case the air bag ruptures, and since some air bags are designed to deflate by releasing the gas within the confines of the interior of the vehicle. Otherwise, toxic or noxious gas may injure or cause illness to the occupants. For example, the release of too much carbon monoxide could cause illness and even be deadly to the occupants. These toxicity requirements are controlled by certain specifications required by the automotive manufacturers. For example, a typical automotive requirement is that an inflatable air bag system meet certain specifications for a 100 cubic foot compartment. These toxicity specifications are set by health requirements and one reference which is helpful in defining those requirements is OSHA workplace breathing air standards. Another reference is the American Conference of Governmental Hygienists' Allowable Limits for Short Term Exposure Levels for the Workplace.
In addition, the gas-generating composition may be highly toxic or unstable requiring special handling during the manufacturing process and creating disposal problems at the end of the useful life of the vehicle. For example, raw sodium azide which is used as the gas-generating composition in most airbag inflators today has a relatively high toxicity which creates handling problems during the manufacturing process.
Other considerations include that the gas and any other materials, for example solid particles, released into the air bag must meet energy transfer restrictions so that it will not burn or deteriorate the integrity of the air bag. Insuring that the energy and materials transferred during the inflation event do not burn, puncture or deteriorate the bag, protects the occupants from injury and insures proper bag inflation.
Packaging restrictions add a further design consideration in the development of passive air bag inflators. For example, weight and size are primary factors in determining the suitability of vehicle inflator designs. Weight reduction translates into fuel economy improvements and size reduction into styling and design flexibility. For styling reasons and customer-acceptance, and so as not to interfere with the occupants' movement, comfort or the driver's line of vision, it is desirable to arrange the inflator so as not to be obtrusive, and yet have it positioned so that it effectively accomplishes its intended task. In order to accomplish these styling, customer-acceptance and engineering design parameters, the inflator must be capable of being packaged in a compact manner. For example, it is desirable to package the inflator in an air bag module which fits with the hub of the steering wheel while still allowing the use of the vehicle's horn by depressing any part of the steering wheel hub and while additionally allowing the use of the numerous control switches on the steering column. It is further advantageous for side impact bags to package the inflator and air bag module between the exterior door panel and the trim or panel in the interior of the door.
The emphasis on weight reduction for the purpose of fuel conservation in motorized vehicles, and the recent development of passenger air bags, rear-seat occupant air bags, side-impact air bags, seat-belt air bags and knee-bolster air bags as well as the contemplated use and development of air bags in the A and B pillars of vehicles and other small bags of 1 to 30 liters of volume, have created the need and demand for a light and compact inflation system.
There are basically two methods or systems which are employed to supply the gas in air bag restraint systems. In one method, the inflating gas is provided as a compressed gas stored onboard the vehicle within a pressure vessel. In the second method, the bag is inflated by igniting a pyrotechnic gas-generating propellant composition and directing the resultant gaseous combustion products into the bag. These two methods create three categories of inflators, the first relies solely upon a pressurized reservoir of gas, the second upon burning a combustible propellant to generate all of the gas to fill the air bag, the third upon a combination of the two described methods to inflate the air bag, and is known in the art as a hybrid inflator.
The first method requires a reservoir of gas stored onboard the vehicle at a very high pressure which is discharged into the bag immediately upon sensing the impact. In order to inflate the vehicle occupant restraint bag in the required time of 0.010 to 0.065 seconds, that is to attain a fill volume rate of at least about 900 liters per second and preferably approximately 3,000 liters per second, a relatively large reservoir of gas at pressures of 3,000 pounds per square inch ("psi") is stored in a pressure vessel. To open the pressure vessel in the short time interval required to inflate the air bag, explosive actuated arrangements are employed for bursting a diaphragm or cutting through a structural portion of the reservoir.
In the second method, a pyrotechnic gas generator having an ignitable and rapid-burning gas-generating propellant composition burns to produce substantial volumes of hot gaseous products which are directed into the inflatable bag. These gas generators must withstand thermal and mechanical stresses during the gas-generating process. Specifically, the gas-generating propellant ignites, combusts and burns at elevated temperatures and pressures which require the casing (pressure vessel) surrounding the gas-generant to be capable of safely withstanding these elevated pressures at a specified safety factor. These strength requirements result in a large, bulky and heavy inflator typically of toroidal shape, for driver-side applications.
Typically, there is a center chamber in these gas-generating inflators used for pyrotechnic ignition-enhancers and auto-ignition materials ("AIM"). The center chamber is concentrically surrounded by one or more separate chambers in fluid connection with each other and with the center chamber. The concentric chamber typically contains the main propellant charge and filter. Structural members divide the concentric chambers and are connected to the outer pressure vessel usually by means of a weld, rivet, screw thread or other mechanical fastening means. These structural members typically form a central post construction for the pressure vessel which adds to the strength, weight and size of the inflator.
Pyrotechnic compositions typically include a fuel and an oxidizer. Because gas-generants, including sodium azide which is used today in most passive restraint systems, and most pyrotechnic oxidizers typically produce significant amounts of solid particulates, filters are typically incorporated into the inflator to separate the hot particles from the gas prior to exhausting the inflating gas into the air bag. The solids produced during combustion are separated from the gas stream to prevent the particles from rupturing the bag and injuring occupants. In addition, as described above, it is important to produce an inflator gas having a temperature which is sufficiently low to avoid burning or deteriorating the integrity of the air bag or belt. However, gas-generants which burn faster and better at lower temperatures tend to produce significant quantities of particulates making filters all the more important when using these low temperature-burning gas-generating materials.
In addition, the filters in prior art inflators also acted as a heat sink to reduce the temperature of the gaseous products filling the air bag. The filters which are usually made from metal are helpful in absorbing the heat from the gaseous products and often provide a torturous path for the gaseous products to travel which further absorbs the energy of the gaseous products in order to protect the integrity of the air bag or belt.
The structural members forming the central post construction and filters in these gas-generating inflators add weight, complexity, cost and bulk to the inflators. For the reasons described, decreasing weight and size are desirable in the design of automotive inflators for passive air bag restraint systems. Although there have been inflator designs which have deviated from these typical designs, they still include the disadvantages of filters or central post construction or other bulky designs. For example, U.S. Pat. No. 5,556,130 describes a pyrotechnic inflator having a generally cylindrical pressure vessel formed of sheet metal which includes an annular filter having a plurality of convolutions of metal screen having decreasing mesh size as it progresses to the outside of the filter where it abuts against the pressure vessel walls. U.S. Pat. No. 4,923,212 discloses a pyrotechnic inflator having a domed pressure vessel which includes an annular filter abutting against the pressure vessel walls. In both of these patents, the filter adds to the weight, size and complexity of the inflator design.
U.S. Pat. No. 5,551,725 discloses several inflator embodiments. Two embodiments include a cylindrical tubularly-shaped pressure vessel which is relatively bulky, having a length substantially larger than its diameter and which relies on the ignition-enhancing materials being blown into the portion of the pressure vessel containing the gas-generant composition to provide quick and efficient mixing and burning with the main gas-generant. This design disadvantageously relies on a bulky, large and inconveniently shaped pressure vessel which is difficult to use in many applications and is not sized to be a drop-in replacement for existing inflators. The other embodiment of the invention disclosed in U.S. Pat. No. 5,551,725 relies on a toroidally-shaped pressure vessel having a structural central post structure which disadvantageously adds to the bulk, weight and complexity of the inflator.
The third category, the hybrid inflation system, utilizes a gas-generating propellant composition and a pressurized medium to meet the requirements of air bag restraint systems. As such, a hybrid inflator suffers many of the drawbacks of the other two categories of inflator designs, and is often of complex design. These hybrid systems typically store pressurized gas at about 3,000 psi. In operation, they burn gas-generating propellant grains to produce heated gas as well as to heat the stored gas. Hybrid inflators produce less solid particles since less solid particulate producing gas-generant can be used to obtain the same inflator gas output. In addition, the stored pressurized gas cools the gas which flows into the inflator.
The combination of greater condensation of solids within the inflator and the reduction of solids produced allows some hybrid inflators to operate without filters. Current driver-side hybrid inflators are toroidal in shape with a center chamber typically used for the hybrid heater assemblies surrounded by one or more separate chambers in fluid connection typically containing the pressurized gas. Structural members divide the concentric chambers and are connected to the outer pressure wall usually by weld, rivet, screw thread or other mechanical fastening means. Hybrid inflators have a number of drawbacks: first they are more complicated and have more parts. Second, there is higher cost associated with more parts and the additional handling and assembly operations. Third, they are larger and heavier because the inflator energy is in part stored as a pressurized gas rather than a solid and fourth, they have decreased reliability resulting from storing the pressurized gas over the lifetime of the vehicle.